Research On The Bias Control Technology Of Electro-Optic Modulator In RoF System

Combining the advantages of wireless and optical communication techniques, radio over fiber(Ro F) technique has become a promising solution for future broadband wireless access in the millimeter-wave band. Mach-Zehnder electro-optic modulator(MZM) based on lithium niobate(Li Nb O3) is the core device of the Ro F system and is widely used in optical communications due to its remarkable advantages, such as the low optical loss, large bandwidth, and chirp-free or tunable chirp. However, the materials and structures of the MZM make it susceptible to environmental changes such as temperature, stress, or mechanical vibration. These will cause slow drift of the optimum bias point in MZM, and eventually result in non-linear distortion, error rate increase, system instability and performance reduction of the Ro F system. Therefore, locking the optimum bias point of MZM is the key to guarantee the Ro F system performance. In this thesis, the arbitrary-point bias control technique for the MZM in Ro F system are investigated, and the auto bias control scheme that utilizes pilot tone to lock the cascaded MZMs at arbitrary points are analyzed in detail.Firstly, the basic principle of the electro-optic modulator is introduced, as well as the nonlinear characteristic and the phenomena of bias drift. Influence of the temperature fluctuation on bias points was analyzed, and the influence of the bias drift on the performance of Ro F system was summarized. Secondly, based on the solution on pilot tone locking in common working point bias control, a bias control scheme that can lock the modulator at any operating point using a pilot tone was demonstrated. Then the algorithm flow of the ratio between the first harmonic of the pilot tone and the average optical current is explained. The system modeling and algorithm emulation show that the locking error around the maximum or minimum transmission points is within ±0.5 degrees, and the locking error near the quadrature point is within ±10. Thirdly, the modulation principle of cascaded MZMs is analyzed and the millimeter wave generating schemes based on cascaded MZMs is summarized. Based on the above work, a bias control solution for the cascaded MZMs is proposed. The system modeling and algorithm emulation show that when the two MZMs are both working at the maximum transmission points, the locking error is within ±0.50. When the two MZMs are working at 1350 and 1650, respectively, the locking error is within ±10. Finally, the following modules for the cascaded MZM bias control scheme are designed: pilot tone generation, photoelectric detection and preamplifier, low pass filter and bias DC voltage. The circuit simulation shows that all of the modules can satisfy the requirement of the cascaded MZM bias control scheme.